A mercury switch has heretofore been used to make or break the electric circuit by a contact in which the surface is wetted with mercury. Mercury permits the conduction of electricity at a contact point in the mercury switch. Accordingly, the mercury switch exhibits very small contact resistance and increased stability. The fixed contact of the mercury switch is usually formed through the steps mentioned below. Namely, an oblong contact material is welded onto the flat surface of a magnetic pole which supports the fixed contact point. Then, the contact material is formed in the shape of a contact point by press working, followed by chromium plating over all the surfaces. Thereafter, chromium on the contact surface of the contact point is removed by polishing by electric discharge. Finally, the remainder of chromium is oxidized to form chromium oxide on the surfaces other than the contact surface. Therefore, the contact surface can be easily wetted with mercury, but other portions are not easily wetted with mercury.
The fixed contact can also be formed by the following method. That is, the flat magnetic pole is plated with chromium throughout all the surfaces, and a granular contact material is welded onto the magnetic pole.
The contact point prepared by the former method features a small contact area. However, since the former method polishes the contact surface by using an electric discharge, the manufacturing cost becomes expensive, which inhibits mass-production.
With the latter method, the step for polishing the contact point can be eliminated. However, since the magnetic pole tends to be wetted with mercury up to the side surface of the contact point, mercury tends to adhere in large amounts to the contact point. Such a large amount of mercury tends to easily come into contact with an armature which is a moving contact. Further, once contacted, the moving contact can not easily be separated away from mercury. To obtain a smooth contact operation, thereore, it is necessary to increase the gap between the contact points. However, the increased gap decreases the sensitivity when the mercury switch is adapted to the relays.